Citrus does not come true from seed.

(avoids temptation to mark quotation with [lime] tags)

The wonderful Oxford Companion To Food goes on about this, too. Basically, in many highly-bred plants, the borderlines between species are not as clear-cut as "this is a giraffe, but that's a hamster". If you think about (say) limeness as a recessive gene that's carried by many oranges, then there will be a certain number of lime trees every time two oranges pollinate.

The same, IIRC, happens with brassicas, where a huge variety of cultivars (cabbages, kale, brussels sprouts, oilseed rape, cauliflowers, broccoli/calabrese) have been bred from the relatively small genetic raw material of the seakale. "Although consumers can easily distinguish broccoli and cauliflowers," says OCF, "botanists have great difficulty".

Peter

PS. I read somewhere that all the world's domesticated hamsters are descended from one pregnant female caught one day in Egypt. Anyone know whether that's true?